1. Field of the Invention
The present invention relates to a short arc discharge lamp which is used as a light source for the lighting optical system such as the exposure device and UV spot cure, said exposure device serving to print electron circuit patterns on semiconductor wafers.
2. Description of the Related Art
The short arc discharge lamp including the super-high pressure mercury lamp, xenon mercury lamp and the like usually has a bulb made of quartz glass and a pair of caps attached to the both ends of the bulb to keep the bulb air-tight. The center portion of the bulb is shaped like an egg. A pair of anode and cathode electrodes are mounted to oppose each other. A sight interval is provided between the anode and cathode electrodes to discharge short arc between them.
The short arc discharge lamp is a ultraviolet lamp capable of discharging small arc like a spot light source. Therefore, the short arc discharge lamp is combined with optical systems, which are provided with reflectors and the like, and used as various precise industrial means.
As disclosed in preliminarily-opened Japanese Patent Application Sho No. 60-57930, the exposure device for printing electron circuit patterns on semiconductor wafers has the short arc discharge lamp as its light source. The exposure device further includes a reflector for refecting and focusing light emitted from the discharge lamp, a plane mirror for reflecting light in different direction reflected by the reflector, a lens for focusing light reflected by the plane mirror, a photomask through which light focused by the focusing lens passes, and a semiconductor wafer onto which light passing through the photomask is focused and irradiated. The reflector has a quadric surface of revolution. The short arc discharge lamp is located substantially in the focus point of this reflector. The semiconductor wafer is coated with ultraviolet-sensitive resin.
A pattern of the photomask is printed on the surface of the semiconductor wafer by ultraviolet emitted from the short arc discharge lamp. Light is irradiated several times to expose one of the semiconductor wafers and high DC current is supplied to the discharge lamp only at the time when light is irradiated.
As disclosed in another preliminarily-opened Japanese Patent Application Sho No. 63-34897, the short arc discharge lamp is used for the projector. Current is applied to the short arc discharge lamp of the projector, synchronizing with the shutter of the projector. This current is converted to high level one only when the shutter is opened, and supplied to the discharge lamp. The short arc discharge lamp emits light of high brightness only when high level current is supplied to the lamp.
Industrial working machines have been asked these days to have high working accuracy, and they have also been asked to have high efficiency to enhance productivity. Super LSIs have been extremely highly integrated and the device for exposing semiconductors is thus asked to have high efficiency in the field of manufacturing semiconductors, too.
The strength or brightness of light on the surface of the semiconductor wafer is increased to enhance the exposing efficiency of the exposure device. Efficiencies of such means as the reflector and focusing lens are enhanced, while the output of the short arc discharge lamp is raised to increase the strength or brightness of light. Such means as the reflector has been completely improved, having an efficiency higher than a certain level. However, the short arc discharge lamp must be still improved. Conventionally, the strength of light increases to enhance the exposing efficiency by the short arc discharge lamp. A discharge lamp having a large light output has been thus developed and used as means for enhancing illuminance efficiency. A discharge lamp having a lamp input of 1 KW is therefore used instead of the amp having a lamp input of 500 W to shorten exposure time to half. The industrial machines provided with the discharge lamp enhance their efficiency in this manner.
In the case of the conventional short arc discharge lamp, however, power inputted become large when the output of the lamp is made high. Therefore, the discharge lamp cannot avoid its becoming large in size to release larger heat by means of increasing input power. When the discharge lamp is made large in size, its bulb also becomes large-sized and the diameter of arc discharged is also made large.
When the large-sized short arc discharge lamp is used in the exposure device which is asked to have high accuracy, light cannot be focused on the focusing point because the diameter of arc discharged is large. The light focusing ability of the exposure device is thus lowered and light is diffused on the surface of the semiconductor wafer. Therefore, the strength of light is not increased but lowered.